Orientating optical fibres

ABSTRACT

A method and apparartus for setting the orientation of an optical fibre (6) having at least one planar section extending along its length comprises a light source (3) for generating a beam of light which impinges on the fibre (6). Light reflected by the fibre is detected by a light detector (5) and the fibre is rotated by means of a tube (1) into which it is inserted while the intensity of the detected light is monitored. When the intensity becomes a maximum then the planar face is facing in a known direction.

The invention relates to methods and apparatus for setting theorientation of an optical fibre having at least one planar sectionextending along its length.

Optical fibres have recently been developed which, instead of having aconventional circular cross-section, have a semi-circular section. Thesefibres are known as D-fibres. D-fibres provide a relatively simple meansof access to the fibre core and can form the basis of severalfibre-based devices.

In order to fabricate a device incorporating such a fibre, it isnecessary to identify the planar section or flat on the fibre. Inpractice, identifying a parallel flat section on a fibre of only 125micron diameter and orientating it correctly is not simple. One methodwhich has been used in the past is to allow the D-fibre to bend since,because of its cross-section, this fibre is forced to orientate itselfsuch that the flat lies to the inside or outside of the bend. A previousmethod of identification simply involved holding a length of fibrehorizontally and allowing one end to droop down. This resulted in atendency for the flat to lie up or down and by careful scrutiny of thelight reflected from the flat surface it was possible to identify theflat. This method has several disadvantages: it is very operatordependant, unreliable, difficult to see the reflected light, and evenwhen the flat has been identified the fibre still has to be rotated intothe correct orientation for mounting.

In accordance with the one aspect of the present invention, a method ofsetting the orientation of an optical fibre having at least one planarsection extending along its length comprises irradiating the fibre witha beam of radiation, monitoring radiation reflected by the fibre, androtating the fibre until the intensity of the monitored radiationreaches a maximum or exceeds a threshold.

In accordance with a second aspect of the present invention, apparatusfor setting the orientation of an optical fibre having at least oneplanar section extending along its length comprises a fibre supportassembly including rotatable means whereby an optical fibre may besupported and rotated; and detection means for irradiating at least oneregion of the fibre with a beam of radiation sensing radiation reflectedby the fibre, and indicating when the intensity of the sensed radiationreaches a maximum or exceeds a threshold.

We have devised a method and apparatus which enables the orientation ofthese optical fibres to be set automatically with a minimum of operatorintervention. The invention makes use of the fact that the optical fibrewill not reflect light equally while it is rotated, the maximum amountof reflection occurring when the radiation beam impinges on the flat.

Typically, the radiation will comprise optical radiation, such asinfra-red radiation generated by a light emitting diode but other typesof radiation are also suitable.

In one preferred embodiment, the method comprises a preliminary step ofimparting a bend in the optical fibre. This has the advantage ofrestricting the location of the planar section, in the case of aD-fibre, to one of two orientations and thus reduces the uncertaintyinvolved in locating the planar section. The rotating means preferablycomprises a rotably mounted curved guide. This provides a simple way ofcontrolling the orientation of the optical fibre and at the same time,as explained above, limits the position of the planar section to one ortwo orientations thereby simplifying the setting steps. Furthermore, theorientation will be fixed. Typically, the curved guide will comprise acurved tube similar to a hyperdermic.

The invention is particularly suited to the automatic generation ofoptical fibre sections of known orientation and the method preferablycomprises setting the orientation of each end of a section of opticalfibre to be substantially the same by performing a method according tothe first aspect of the invention, clamping the ends and thereaftersevering the section of optical fibre adjacent to the clamped ends. Thecut section of optical fibre can then be set into an optical device in aconventional manner. Preferably, therefore, the apparatus furthercomprises clamping means for clamping the optical fibre at the desiredorientation.

Preferably, the support assembly comprises bearing means for rotatablysupporting a section of the optical fibre. For example, the opticalfibre may be supported at one end by an air bearing and at the other endby the rotatably mounted curved guide, when provided.

In one example, the detection means is adapted to irradiate two regionsof the fibre spaced apart along its length so as to set the orientationof each of those regions to ensure that the fibre is not twisted.

The invention can also be adapted to set the orientation of an array ofoptical fibres each of which has at least one planar section extendingalong its length in which case the support assembly is adapted tosupport a number of optical fibres, and the detection means and supportassembly are relatively movable so that the orientation of eachsupported fibre can be set in turn. Alternatively, the detection meanscould comprise separate detectors one for each fibre so that relativemovement would not be required.

Typically, in order to be able to set the orientation of closely spacedfibres, the detection means includes a mask positioned between aradiation source and a radiation sensor of the detection means toprevent the radiation sensor sensing radiation reflected by fibres otherthan the one whose orientation is to be set.

In a further preferred embodiment, the apparatus forms part of a fibredrawing plant. Preferably the apparatus is operatively connected withmeans for rotating the optical fibre in such a way that a desired fibreorientation can be maintained. Preferably the means for rotating theoptical fibre form part of the fibre drawing mechanism, more preferablythe means for rotating the fibre also advances the fibre in the drawingprocess.

Although the invention is primarily concerned with the orientation ofD-optical fibres, it may also be applied to optical fibres having morethan one planar section.

Some examples of methods and apparatus in accordance with the presentinvention will now be described with reference to the accompanyingdrawings, in which:

FIG. 1 illustrates graphically the variation in intensity of reflectedlight with the angle of rotation of the optical fibre;

FIG. 2 is a schematic, block diagram of part of one example of theapparatus;

FIG. 3 illustrates a modification to the end of the tube shown in FIG.2;

FIG. 4 illustrates the cross-section of a typical D-fibre;

FIG. 5 illustrates the support assembly for the FIG. 2 example;

FIG. 6 illustrates part of a second example of the apparatus for settingthe orientation of an array of fibres;

FIG. 7 illustrates the resolution of the apparatus shown in FIG. 6;

FIG. 8 illustrates the position of the mask used in the FIG. 6 example,and

FIG. 9 illustrates schematically how fibre orientating apparatusaccording to the invention can be incorporated into a fibre pulling rig.

The apparatus shown in FIG. 2 comprises a hyperdermic tube 1 which hasbeen bent through an angle of about 90° and which is pivoted at one endby a simple bearing 2. An optical source, such as a light emitting diode3 is mounted in a support block 4 adjacent to a light detector 5 such asa photodiode. The block 4 is positioned beneath a D-optical fibreadjacent to where it enters the hyperdermic tube 1. Typically, thediameter of the D-fibre is 125 microns while the diameter of the tube isabout 0.75 mm.

The light source and sensor 3, 5 are coupled with the electronics 7including a power source 8 coupled with the light source 3 and avariable gain amplifier 9 coupled with the light sensor 5. The amplifier9 amplifies the output from the light sensor 5 and this drives a lightemitting diode indicator 10 whenever the output signal from theamplifier 9 exceeds a threshold T.

In use, one end of the fibre 6 is inserted into the tube 1 is pushedinto the bent portion. This orientates the fibre 6 such that the planaror flat surface of the fibre lies to the inside or outside of the bendand retains the orientation when the tube 1 is rotated. As can be seenin FIG. 4, the D-fibre has a generally semi-circular cross-section withthe core region 11 adjacent to the flat 12.

With the tube 1 orientated horizontally as shown in FIG. 2, the flat 12either lies face down or face up. If the flat is face down then thelight transmitted from the source 3 will be reflected by the flat 12 andbe received by the sensor 5 with a sufficient intensity to cause the LEDindicator 10 to light. If the flat 12 is face up, however, only a smallproportion of the light from the source 3 will be reflected onto thesensor 5 so that the received intensity is less than the threshold andthe LED indicator 10 will not light. By rotating the tube 1 within thebearing 2, the fibre 6 will also be rotated and as soon as the flat 12is substantially face down the sensor 5 will detect sufficient light tocause the LED indicator 10 to light and the orientation of the fibre 6will then be known.

The variation in intensity of the reflected light with rotation angle ofthe fibre is illustrated in FIG. 1. It is assumed in FIG. 1 that at 0°the flat 12 will be face down so that the intensity of the lightreceived by the sensor 5 is at a maximum level. As the fibre is rotatedthrough 180°, the intensity will gradually decrease to a minimum andthereafter increase back to the maximum after a full 360° rotation. Thethreshold intensity level T is set empirically so as to correspond tothe fibre being in a satisfactory orientation.

Typically, as in this example, the bore of the tube 1 will be severaltimes the diameter of the fibre 6. In addition, the leading end of thetube may be provided with a belled end 13 to aid insertion of the fibre6 (FIG. 3).

The support assembly for the fibre 6 is shown in more detail in FIG. 5.The support assembly comprises a rectangular support block 14 onto whichthe fibre 6 is clamped using clamp members (not shown) once theorientation of the fibre is set. The end of the fibre opposite to thetube 1 is supported in an air bearing 15. Typically, the fibre will besupplied from a spool after removal of its coating through the airbearing 15, across the support member 14 and into the tube 1. Initially,the orientation of the fibre will be set as explained above adjacent tothe bearing 15 and then the fibre will be clamped at the region 17 tothe support member 14. A similar setting of the orientation of the fibre6 will then be performed adjacent the tube 1 using another detector (notshown) and the fibre will then be clamped at the region 16. At thispoint, the orientation of the section of fibre between the region 16, 17is known and this section of fibre can then be cut or cleaved in aconventional manner and set into the device to be constructed. Theclamps are released and the portion of the fibre downstream of theregion 16 is discarded and then a further length of optical fibre issupplied via the air bearing 15 into the tube 1 to allow the operationto be repeated.

The system sensitivity may be set by adjusting the gain of theoperational amplifier 9, the higher the gain the more tolerant thesystem is to lateral and angular misalignment of the D-fibre. Forexample, with the distance between the fibre and the sensor 5 set to 5mm, the following results for the flat detection were obtained (see FIG.4).

Sensitivity Set to Maximum:

lateral misalignment=±1.65 mm

angular misalignment=±35°

Sensitivity Set to Minimum:

lateral misalignment=±0.25 mm

angular misalignment=±approx 1°.

When assembling arrays of two or more fibres side by side it will benecessary to detect the flat of each fibre in turn without the presenceof the adjacent flats affecting the measurement. To do this thehorizontal resolution must be improved from that obtained by thearrangement shown in FIG. 2. One solution to this is illustrated inFIGS. 6 and 7. In this case, a mask 18 is mounted on the block 4 betweenthe light source 3 and the sensor 5. By adjusting the width d, height h,and spacing of the mask from the array s (FIG. 8) there will only be avery small distance laterally over which the fibre may be placed whereit will be "seen".

In operation, the block 4 is fixed and the array of four, substantiallyparallel fibres 19-22 mounted to a substrate 23 is moved over the blockin the direction shown by the arrow 24. Due to the presence of the mask,each fibre will be seen individually in turn and can be orientated usingthe method previously described.

With this system a resolution as small as ±50 microns has been obtainedwhich is better than that required by individual flats spaced only onefibre width apart as may be seen in FIG. 7.

The orientation device according to the invention finds particularapplication in conjunction with a D-fibre drawing rig, that is duringthe production of D-fibre from a perform, where the orientation of thefibre is monitored and, via a feedback loop, controlled in order toavoid the unwanted incorporation of twist or torsion in the fibre. Builtin torsion is generally undesirable because, when the fibre is cleaved,as for example is normally prior to jointing or splicing, instead if thedesired normal end face an off-normal end face is produced. The angle bywhich the end face is off-normal to the fibre longitudinal axis istermed the "end angle".

End angles of more than 1° or 2° are generally unacceptable as they leadto excessive splice losses. While torsion in conventional circularlysymmetric fibre would similarly lead to undesirable end angles, inpractice, circular section optical fibres do not acquire torsion duringpulling, so not special efforts are required to control it. It may bethat the asymmetrical shape of D-fibres is the root cause of theproblem, but whatever the cause torsion need to be controlled.

A detector head according to the invention can be set up to monitorfibre orientation in the region where the fibre emerges from thefurnace, that is before the point at which the fibre is provided withits primary coating. Advantageously, in place of the LED sourcementioned earlier, a laser, for example a helium-neon laser, is used asthe optical source in the detector head. The use of a laser enables thesource and detectors to be mounted at a significant distance from thefibre being drawn. Conveniently but not essentially, the laser may alsoserve as the optical source for the diameter sensing arrangement of thetype routinely used in conventional fibre drawing. Advantageously, insuch an arrangement beam splitting means maybe provided to directseparate beams or beam portions at the fibre for the separate functions.The beam splitting means may comprise an optical fibre. In such diametersensing a laser beam or possibly two laser beams are arranged to impingeon the fibre being drawn, the beam or beams being orthogonal to thefibre direction. The laser light shining on the fibre creates adiffraction pattern, which appears as a sequence of light and darkspots. The diffraction pattern subtends a greater angle if the fibrediameter decreases, and subtends a smaller angle if the fibre diameterincreases. The diffraction pattern is monitored, the output of themonitoring detector means being used to control, via a feedback loop,the pulling speed of the fibre and hence the fibre diameter. The controlof fibre torsion can be effected by rotating the furnace end of thefibre pulling line or, preferably and more conveniently, by effecting acontrolled rotation of the fibre at the pulling end of the line.Conveniently the conventional pulling head which comprises a capstanwheel or roller, typically of metal, with one or more associated pinchwheels or rollers is replaced by a pulling head in which belts act onthe fibre to provide the pulling force. Such a pulling head mayconventiently comprise a pair of facing drive belts, the direction ofmotion of the two drive belts and the fibre being essentially in line inone plane, but the relative inclination being adjustable so that acontrolled degree of twist may be added to the fibre as it is advancedat a controllable rate. Such pulling heads are in themselves known,being sold as means of adding a controlled degree of twist toconventional fibre in order to give the fibre particular desiredproperties. The orientation detector according to the present inventionprovides a convenient source of orientation information about a movingfibre, which information can be used by the microprocessor or computerwhich controls the pulling head or whole pulling rig to control thelevel of twist built into a D-fibre during production.

We claim:
 1. A method of setting the orientation of an optical fibrehaving at least one planar section extending along its length, themethod comprising irradiating the fibre with a beam of radiation,monitoring radiation reflected by the fibre, and rotating the fibreuntil the intensity of the monitored radiation reaches a maximum orexceeds a threshold.
 2. A method according to claim 1, wherein theradiation comprises optical radiation.
 3. A method according to claim 1the method being implemented as part of a fibre production process.
 4. Amethod according to claim 3 wherein the fibre production processincludes forming the fibre from a heated preform, coating the fibre witha primary coating, and passing the coated fibre through fibre advancingmeans, said beam of radiation being arranged to impinge on the fibrewhile it is advanced and at a point before that at which the fibre isprovided with the primary coating.
 5. A method according to claim 3wherein the orientation of the fibre as it is pulled is initiallyadjusted to a desired orientation, the intensity of radiation beingmonitored, intentional rotation of the fibre being effected subsequentlyduring the pulling process only as necessary to maintain the desiredorientation as determined by the monitored radiation level.
 6. A methodaccording to claim 1 wherein the radiation is visible or infra-red lightfrom a laser source.
 7. A method according to claim 1, furthercomprising a preliminary step of imparting a bend in the optical fibre.8. A method of generating a section of optical fibre of knownorientation, the method comprising setting the orientation of each endof a section of optical fibre to be substantially the same by perfominga method according to claim 1, clamping the ends and thereafter severingthe section of optical fibre adjacent to the clamped ends.
 9. Apparatusfor setting the orientation of an optical fibre having at least oneplanar section extending along its length, the apparatus comprising afibre support assembly including rotatable means whereby an opticalfibre may be supported and rotated; and detection means for irradiatingat least one region of the fibre with a beam of radiation, sensingradiation reflected by the fibre, and indicating when the intensity ofthe sensed radiation reaches a maximum or exceeds a threshold. 10.Apparatus according to claim 9, wherein the detection means includes alight emitting diode for generating the beam of radiation.
 11. Apparatusaccording to claim 9, wherein the rotating means comprises a rotatablymounted curved guide.
 12. Apparatus according to claim 11, wherein thecurved guide comprises a curved tube.
 13. Apparatus according to claim9, wherein the support assembly comprises bearing means for rotatablysupporting a section of the optical fibre.
 14. Apparatus according toclaim 9, wherein the detection means is adapted to irradiate two regionsof the fibre spaced apart along its length so as to set the orientationof each of those regions.
 15. Apparatus according to claim 9, whereinthe apparatus further comprises clamping means for clamping the opticalfibre at the desired orientation.
 16. Apparatus for setting theorientation of an array of optical fibres each of which has at least oneplanar section extending along its length, the apparatus comprisingapparatus according to claim 9, the support assembly being adapted tosupport a plurality of optical fibres, and the detection means andsupport assembly being relatively movable so that the orientation ofeach supported fibre can be set in turn.
 17. Apparatus according toclaim 16, wherein the detection means includes a mask positioned betweena radiation source and a radiation sensor of the detection means toprevent the radiation sensor sensing radiation reflected by fibres otherthan the one whose orientation is to be set.
 18. Apparatus according toclaim 9 wherein the fibre support assembly is part of a fibre drawingplant, the fibre section which is to be orientated extending generallyvertically between preform heating means and fibre pulling means,torsion control means being provided to control the degree of torsionincorporated into the fibre being pulled between the preform heatingmeans and the fibre pulling means, the torsion control means beingcontrollable in response to the output of said detection means. 19.Apparatus according to claim 18 wherein said fibre pulling means andsaid torsion control means are combined.
 20. Apparatus according toclaim 19 wherein the fibre pulling means comprises a pair of drive beltsbetween which the fibre is gripped, an angle between the drivedirections of the two belts being adjustable to adjust the degree oftwist given to the fibre as it is advanced.